In the Wideband Code-Division Multiple Access (W-CDMA) networks developed by members of the 3rd-Generation Partnership Project (3GPP), so-called scrambling codes are used to differentiate between downlink signals transmitted by neighboring cells in the network, as well as to differentiate between uplink signals transmitted to the cells by mobile stations in a given area. Generally, these scrambling codes are assigned to the cells in the network by the network operator, using any of various cell planning tools.
Scrambling codes are also known as pseudo-noise codes and are one of two spreading codes groups used in W-CDMA systems. The other type of code used in W-CDMA system is the channelization code, which is used for channel separation of one transmission from another. Coding of subscriber information is achieved by “multiplying” the transmitted information with channelization and scrambling codes. More particularly, after the channelization codes is applied to user data to map the user data to a CDMA channel, the data stream is multiplied by a code from a group of special binary codes, to distinguish between different transmitters, which are in turn mapped to cells. The code gives a unique user equipment (UE)/base station (BS) identity. This process is referred to as “scrambling” and the codes used for this process are hence called “scrambling codes.” The codes used are selected to produce a low correlation value when correlated with other codes, which provides a good separation between multiple transmission sources.
In a basic W-CDMA network, since all transmitters are on the same frequency there is no need for frequency planning. However, adequate physical separation is required between cells that are using the same scrambling codes. There are 512 unique scrambling codes used in W-CDMA. Hence there is a need to maintain uniqueness of scrambling codes between adjacent W-CDMA cells.
W-CDMA handover decisions are taken by a Radio Network Controller (RNC) based on radio measurement data obtained by the user equipment (UE-3GPP terminology for a mobile terminal or access device) and reported to the network. These measurements are performed to determine the quality (e.g., signal strength) of transmissions from the cell or cells that are serving the UE, as well as of transmissions from nearby cells. The RNC keeps track of neighbor relations between the various cells managed by the RNC; these configured neighbor definitions in the RNC are used to inform the UE of which scrambling codes must be measured. The 3GPP-defined message containing a measurement order from the RNC to the UE has room for 32 IAF (intra-frequency) cells, including the active set cells. The ability for the RNC to transfer neighbor relation information is limited to this number of neighbors. This is an important restriction that needs to be taken into consideration when planning the IAF neighbor relations.
3GPP specifications for W-CDMA operation also require the UE to find other strong cells apart from the ones requested by the RNC. However, the performance requirements for these measurements are less strict in comparison with what is required for the IAF monitored subset.
The RNC and UE communicate cell identities through scrambling codes. Since scrambling codes are re-used throughout a network, it is not a unique identifier. With the configured neighbor definitions, the RNC is able to uniquely identify a cell by verifying that the reported scrambling code is in the list of neighbors. The configured neighbor relations are used to identify the scrambling codes the UE should measure among the 512 possible, when looking for handover candidates in dedicated mode and cell selection/reselection in idle mode.
The scrambling code assigned to each cell must therefore be unique with respect to scrambling codes assigned to other cells having an adjoining boundary with the cell. These codes must be unique to avoid collisions with the neighboring cell's downlink signals.
In current W-CDMA system implementations, the scrambling codes are usually assigned through a manual process by the network operator, using cell planning tools that group and partition the scrambling codes and cluster these groups for macro- and micro-base station deployments. These scrambling code groups can then be assigned on a per cell site basis, to ensure both scrambling code uniqueness and an ability to re-use these groups efficiently.
Since the allocation of the scrambling code is a manual procedure, it is subject to human errors. A failure to maintain unique scrambling codes between adjacent cells could lead to false preamble detection for mobile stations from adjacent cells, and increase inter-cell site interference. Increased inter-cell site interference in turn can lead to a reduction in throughput and/or connectivity issues with mobile stations. In a deployment with thousands of base stations, it is not difficult to envisage a situation where human error may lead to cell site planning issues. Hence, there is a need for improved techniques for ensuring an efficient allocation of the scrambling codes between cell sites, while reducing or eliminating the need for operator intervention.